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Enhanced leachability of gold and silver in cyanide media: Effect
ARTICLE IN PRESS
Available online at www.sciencedirect.com
Minerals Engineering xxx (2008) xxx–xxx
This article is also available online at:
www.elsevier.com/locate/mineng
Enhanced leachability of gold and silver in cyanide media: Effect
of alkaline pre-treatment of jarosite minerals
H. Kasaini a,*, K. Kasongo a, N. Naude a, J. Katabua b
a
Department of Chemical and Metallurgical Engineering, Tshwane University of Technology, Arcadia Campus, P/Bag X680, Pretoria, South Africa
b
Exxaro Technology, R&D, Aqueous Processing, South Africa
Received 15 June 2007; accepted 10 December 2007
Abstract
Composite samples of tailings containing gold (1.35 g/t) and significant amounts of silver (155 g/t) were subjected to batchwise cyanide leaching to assess the feasibility of extracting gold and silver. The tailings are waste solids arising from flotation and leaching operations whereby the flotation product (sphalerite concentrate) is calcined and then solubilised into dilute sulphuric acid solution and
eventually sequestered from the electrolyte by electrowinning. Silver and gold are part of the zinc refinery residue, flotation tailings
and to a limited extent the calcine leach tailings. Mineralogical results showed that composite tailings are refractory in nature (44%
quartz, 17% silico aluminates and 12% jarosites).
The concept of enhancing gold and silver recovery from the tailings focused on firstly decomposing the jarosite minerals by alkaline
pre-treatment and then secondly leaching with cyanide solution. These two steps ensured that free gold and silver found in the zinc refinery residue and in the jarosite minerals could be leached simultaneously. The composite tailings were treated with Ca(OH)2 solutions and
then heated to 90 °C for 2 h to decompose the silver-bearing mineral (Ag,PbFe3(SO4)2(OH)6). The alkaline pre-treated tailings were then
subjected to cyanide leach tests at different NaCN dosages (2.5–10 kg/t) and particle size (96–200 lm). Without an alkaline pre-treatment
stage, leach efficiencies achieved were 41% and 25% for gold and silver, respectively at 40 °C and 8 h mixing time. But, better leach efficiencies (55% for Au, 81% for Ag) were achieved after the feed was pre-treated with Ca(OH)2. The leaching mechanism of gold was
explained by the shrinking sphere model denoted by surface chemical reaction.
Ó 2008 Published by Elsevier Ltd.
Keywords: Tailings; Cyanidation; Leaching; Jarosites; Pre-treatment; Precious metals
1. Introduction
Presently, about 80% of zinc produced globally is
extracted by roasting sphalerite concentrates followed by
leaching the calcine in dilute sulphuric acid solution and
eventually recovering zinc by electrowinning. However,
roasting of sphalerite concentrates at high temperatures
leads to conversion of haematite minerals (Fe2O3) to franklinite solids (zinc ferrite (ZnO Fe2O3)). The zinc ferrite particles in the calcine material carry a significant fraction of
undesirable iron, which is leached into solution together
with zinc. When iron is precipitated out of the calcine leach
*
Corresponding author. Tel.: +27 012 382 6391; fax: +27 012 382 6275.
E-mail address: [email protected] (H. Kasaini).
solution, significant amount of zinc associated with ferrite is
co-precipitated. Gold and silver in the calcine product
respond poorly to sulphuric acid leaching and therefore
are lost to the leach residues. Furthermore, zinc residue from
the electrowinning cells usually contains gold and silver.
Owing to low grades and fine structure of the gold/silver/zinc residue, recovery of metals by leaching presents
many advantages over physical beneficiation or pyrometallurgy. It was reported (Jackson, 1986) that regrinding of
the residue solids could liberate the gold particles from
flotation tails, however, extreme fineness (d80 averages
45 lm) of the feed material could lead to loss of valuable
metals when gravity separation or wet high intensity magnetic separation (WHIMS) processes are used to upgrade
metals in solids. Several researchers explained that the loss
0892-6875/$ - see front matter Ó 2008 Published by Elsevier Ltd.
doi:10.1016/j.mineng.2007.12.005
Please cite this article in press as: Kasaini, H. et al., Enhanced leachability of gold and silver in cyanide media: Effect ..., Miner. Eng.
(2008), doi:10.1016/j.mineng.2007.12.005
ARTICLE IN PRESS
2
H. Kasaini et al. / Minerals Engineering xxx (2008) xxx–xxx
of valuable metals (zinc/silver/gold) during physical beneficiation was caused by the fact that the zinc/silver/gold residue was too fine and acted as part of the fluid and hence
the poor recoveries of gold and silver.
According to Mineralogical data, quartz was identified as
main component (44.83%) in the sample with minor
amounts of anglesite and magnetite. Jarosite phase accounts
for 12.21% of the weight of the lead/silver residues (Table 1).
The amount gold averaged 1.34 g/t. The purpose of this
study is to develop a feasible process for extracting gold
and silver from the composite tailings containing zinc refinery residue, calcine leach residue and flotation tailings by
cyanidation. Cyanide leaching is predominantly used to
extract gold from the gold ores. Recently, concerns about
cyanide toxicity (Adams, 1998), its relatively slow reaction
kinetics and lack of selectivity has prompted researchers to
seek alternative lixiviants such as thiourea, thiosulfate and
aqua regia (Jackson, 1986; Habashi, 1986; Grosse et al.,
2003). Notwithstanding the efficacy of the above lixiviants,
cyanidation of gold is still the most elegant technique available for gold dissolution on the basis of production efficiency and reaction kinetics (Prasad et al., 1991). Other
researchers have provided useful mathematical models
which describe the leaching kinetics of gold in cyanide solution (Crundwell and Godorr, 1997; McLaughlin and Agar,
1991) and acidified thiourea solution (Gabra, 1984).
1.1. Direct cyanidation of gold
Eqs. (1) and (2) below describe the well-known chemical
reactions for gold cyanidation in the presence of oxygen.
Firstly, at low cyanide concentration hydrogen peroxide
and sodium hydroxide are produced, while at high cyanide
concentration only sodium hydroxide is produced
2Au þ 4NaCN þ O2 þ 2H2 O
! 2NaAuðCNÞ2 þ 2NaOH þ H2 O2
ð1Þ
4Au þ 8NaCN þ O2 þ 2H2 O
! 4NaAuðCNÞ2 þ 4NaOH
ð2Þ
The rate of metal dissolution is twice the rate of O2 consumption and half the rate of cyanide consumption. Eqs.
(3) and (4) illustrate the rate equations for O2 and CN dissolution, respectively,
ð3Þ
ð4Þ
refers to ratio between species diffusivity and
where DCN
d
boundary film thickness. At steady state conditions Eqs.
(3) and (4) are similar, thus
4¼
DCN ½CN DO2 ½O2 ð5Þ
Taking into account the entire surface area, A, of particles
immersed in solution and the fact that A is summation of
A1 and A2, the overall rate of oxygen and cyanide dissolution is expressed by the equation below:
Rate ¼
2A DO2 DCN ½CN ½O2 dfDCN ½CN þ DO2 ½O2 g
ð6Þ
At high cyanide concentration, the rate of metal dissolution is four times higher than the consumption of O2 and
half the rate of CN consumption. In a similar manner,
the rate of oxygen and cyanide consumptions can be
evaluated.
In practice, the effective leaching of gold minerals
requires sufficient agitation and concentrations of oxygen
and cyanide. Several researchers (Habashi, 1992; Health
and Rumbal, 1998; Sandra and Gamini, 2004) have shown
that in laboratory experiments the ratio CN:O2 should be
kept above 6 for effective dissolution of gold although little
information is made available about the limiting conditions
imposed by oxygen and cyanide diffusivities. Recently, in
most industrial gold leaching operations oxygen is added
to the feed slurry through transporting pipes to ensure
maximum diffusion into the solution by taking advantages
of surface shear forces in pipes as well as practicing multimixing configuration of slurries with leach liquor. Also,
excess cyanide solution is added to override the effects of
preg-robbing side reactions between free cyanide ions and
2
sulphide ions (HS, HSO
3 and S ).
In this preliminary study of leaching gold minerals from
tailings, excess cyanide solutions were used in the reactor.
The oxygen supply line was kept at 1 bar.
1.2. Shrinking sphere model
Several authors have described initial leaching kinetics
of precious metals at different temperature by using the
shrinking sphere model denoting surface chemical reaction
(Patino et al., 2003) (Rastas et al., 1990):
Table 1
Mineralogical composition of lead/silver residual tailis
Quartz , SiO2 (%)
Jarosite (argento and plumbo jarosite)
Ag,PbFe3(SO4)2(OH)6 (%)
Potassium, hydronium silico aluminates (%)
Bassanite, CaSO4 0.5H2O (%)
Gypsum, CaSO4 2H2O (%)
Anglesite, Pb(SO4) (%)
Magnetite, Fe3O4 (%)
Galena, PbS (%)
2A1 DO2 ½O2 ¼ k 1 ½O2 d
1 A2 DCN ½CN ¼ k 2 ½CN Rate ¼
2
d
Rate ¼
44.83 ± 2.94
12.21 ± 1.95
11.20 ± 3.30
10.15 ± 2.13
7.26 ± 1.38
5.46 ± 0.66
3.35 ± 0.87
–
2
dX 3ð1 X Þ3
¼
dt
s
or
dr
1
¼
ds
s
ð7Þ
where X = degree of extraction and s = time taken to complete oxidation
1
r ¼ ð1 X Þ3 ¼
d
d0
ð8Þ
Please cite this article in press as: Kasaini, H. et al., Enhanced leachability of gold and silver in cyanide media: Effect ..., Miner. Eng.
(2008), doi:10.1016/j.mineng.2007.12.005
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H. Kasaini et al. / Minerals Engineering xxx (2008) xxx–xxx
d
d0
ratio of diameter at time t, and initial mineral (metal)
diameter. Linearization of the rate equation proceeds as
follows:
1
1 ð1 X Þ3 ¼ k s t
ks ¼ V m K Q
C nA
r0
ð9Þ
ð10Þ
where ks is the apparent reaction rate constant; Vm: molar
volume of the solid; CA: concentration of reactant; n order
of reaction; KQ: chemical constant; t is the leaching time; X
the fraction of gold dissolved;r0: initial radius of particle.
A plot of s10 against the reciprocal of temperature T10 provides more information on the activation energy (Ea) of
reactive oxidation.
2. Experimental
2.1. Materials
Residual solids containing (lead/silver/iron) were drilled
out of the tailings dam at specific locations and placed in
plastic bags. Analytical grade NaCN solution was used as
lixiviant. The composition of the tailings samples, as determined by X-ray diffraction, yielded the following results:
44% quartz, 17% silico aluminates and 12% jarosites. The
rest are traces of zinc, gold and silver minerals.
2.2. Procedure
Batchwise leach tests were performed in 1-l-baffled glass
vessel equipped with five-glass necked ports to hold the pH
electrode, DO probe, thermometer and sampling points or
reagent addition. To keep a constant leach temperature,
the glass reactor was placed in a thermostated bath. At
the beginning of each test, the reactor was charged with
about 1000-ml-distilled water. The reactor was then
brought to a desired temperature after which a calculated
weight of a sample was introduced to a desired % solids
level. Predetermined mass of alkaline solution (lime) was
added and agitation was initiated. After 2 h of alkaline
pre-treatment at 90 °C, the slurry was cooled up to 40 °C.
During cyanidation, the reactor temperature was controlled at 40 °C. Agitation started immediately after an
appropriate volume of 1 M NaCN was added to the reactor under well-aerated conditions (1.1 bar). At predetermined intervals, 20 ml samples were withdrawn from the
leach pulp and the slurry samples were filtered by using
Millipore membrane (0.2 lm). The resultant filtrates were
analysed for solubilised gold and silver and occasionally
for copper, zinc, sulphur, iron and lead, using ICP–MS
(inductively coupled plasma–mass spectrometer).
Throughout the cyanidation process, dissolved oxygen
levels varied between 5.5 and 8.5 mg/l, as measured with
the Syland Dissolved Oxygen Meter and Probe for effective
gold leaching, but due to the presence of silver in the feed,
excess cyanide solution was used. Although the optimum
3
ratio of [CN]/[O2] has been reported as above 6 (Habashi,
1992), excess CN concentration is used in gold leach to
offset the reagent losses due to side reactions that consume
CN and O2 (Sandra and Gamini, 2004). In this study, a
ball mill was used to re-size the particles below 75 lm. Conditions for pre-treatment and leaching are summarised in
Table 2.
3. Results and discussion
The samples from the tailings dam were screened to
determine the size distribution. Table 3 shows the particle
sizes and corresponding cumulative weights. Approximately 79% of the material reported to the 75 lm
fraction.
Table 4 provides information about the metal composition of the solid samples. Although gold and silver are the
target metals, there are significant amounts of zinc and lead
in the feed.
3.1. Pre-treatment of feed with lime: effect on leaching
efficiency of gold and silver
Fig. 1 illustrates the effect of the alkaline pre-treatment
stage. When the tailings are leached without the pre-treatment stage, 41.2% Au and 25.2% Ag were extracted in 8 h
of contact time. The poor results were attributed to the
refractory nature of jarosite minerals present in the lead/silver residue. Alkaline decomposition of the sample followed
by cyanide leachin showed better recoveries of gold
(55.69%) silver (80.17%).
Table 2
Pre-treatment of feed with lime and leach conditions
Conditions
Alkaline treatment
Cyanide leaching
Temperature (°C)
Treatment period (h)
pH (initial, final) (–)
Mass of feed (g)
Initial volume (ml)
Stirring speed (rpm)
DO levels (ppm)
Head grade
90
2
3–11
250
1000
220
–
Au,1.35 g/t; Ag,155 g/t
40
8–24
10.5–11.0
250
1000
220
5.5–8.5
Table 3
Particle size distribution
Sieve size
(lm)
Retained
(%)
>212
150
106
75
45
38
<38
0.98
6.69
6.18
7.55
10.54
12.58
55.20
Total
100.00
Cumulative retained
(%)
Cumulative passing
(%)
0.98
7.67
13.86
21.41
31.95
44.80
100.00
99.02
92.33
86.14
78.59
68.05
55.2
0.00
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(2008), doi:10.1016/j.mineng.2007.12.005
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H. Kasaini et al. / Minerals Engineering xxx (2008) xxx–xxx
3.2. Effect of lime addition on gold and silver extractions
Table 4
Chemical composition of feed
Element
Concentration (%)
Element
Concentration (%)
Au
Ag
Zn
Pb
Cu
Fe
CaO
Mn
1.34 g/t
155 g/t
2.21
5.38
0.161
7.46
4.84
0.275
Ni
K2O
Co
Sb
SiO2
Stot
Al2O3
Ti
0.009
0.985
0.004
0.009
49.4
6.50
4.64
0.077
100
Au (Direct cyanidation)
Ag (Direct Cyanidation)
Au (Alk Digestion & Cyanidation)
Ag (Alk Digestion & Cyanidation)
% Extraction
80
60
40
20
0
0
2
4
6
8
10
Time (h)
Fig. 1. Comparison of % extraction of gold and silver with cyanide
solution before and after alkaline pre-treatment.
Researchers (Berezowsky et al., 1990) have proposed the
following reactions to describe the decomposition of the
H3O–Pb–Ag jarosite species with lime:
H3 OFe3 ðSO4 Þ2 ðOHÞ6 þ 2CaðOHÞ2 þ 2H2 O
! 3FeðOHÞ3 þ 2CaSO4 2H2 O
ð11Þ
Several pre-treatment tests were conducted at different
lime dosages while temperature and percent solids
remained constant (90 °C, 25% solids). The residence time
at the alkaline pre-treatment stage was 2 h followed by 24 h
of cyanide leaching at 40 °C. Gold extraction did not
increase significantly with high lime dosage. Lime dosage
of 100–150 kg/t yielded 65.1–68.2% recovery of gold as
shown in Fig. 2. In fact, further addition of lime beyond
150 kg/t severely impacted on gold recovery. Optimum
dosage was found to be 100 kg/t lime for both gold
(Fig. 2) and silver (Fig. 3).
Table 5 provides a summarised information about the
leach results for gold. Lime over dosage or under dosage
impacted negatively on gold recovery. Gold recovery
peaked at lime dosage of 100 kg/t.
Table 6 shows a similar leach profile to gold. The optimum lime dosage was found to be 100 g/t for effective
leaching of silver.
Alkaline digestion prior to cyanide leaching yielded better leaching results than direct cyanidation process. Gold
recovery averaged 68.17% and silver recovery was 89.80%
with alkaline pre-treatment (Figs. 2 and 3). Since this technique has shown promising results, it was decided to undertake a series of alkaline digestion prior to cyanidation tests
to define other process parameters such as particle size,
cyanide concentration and DO levels, reaction temperature
and contact time.
The lead/silver residual tailings contained about elemental sulphur (6.5%), lead and silver salts and precipitated iron
oxide species, including basic iron sulphates and jarosites.
Sulphide minerals act as preg-robbing agents. Sulphur
may dissolve in cyanide solution to form thiocyanate or
may react with oxygen to form sulphide and sulphate. Small
amounts of sulphide ions, i.e., 0.5 mg/l, (Moussoulos et al.,
1984; Liu and Yen, 1995) can inhibit gold dissolution. However, the ionisation and speciation of sulphides in solution
PbFe6 ðSO4 Þ4 ðOHÞ12 þ 4CaðOHÞ2 þ 8H2 O
80
ð12Þ
AgFe3 ðSO4 Þ2 ðOHÞ6 þ 2CaðOHÞ2 þ 4H2 O
! 3FeðOHÞ3 þ AgOH þ 2CaSO4 2H2 O
60
ð13Þ
XRD results on the leach residue after alkaline pre-treatment and cyanide leaching, showed significant change in
the amount of jarosite species compared to the head sample: from 12.2% to 1.5% of jarosite species (argento and
plumbo jarosite) in the material. Lime digested the jarosite
species and liberated silver following the above chemical
reactions. The silver is then amenable to recovery by cyanidation according to the equation below:
AgOH
ðsÞ þ 2CNðaqÞ ! AqðCN Þ2ðaqÞ þ OH
ð14Þ
% Au Extraction
! 6FeðOHÞ3 þ PbðOHÞ2 þ 4CaSO4 2H2 O
40
150 Kg/t Lime
20
100 Kg/t Lime
60 Kg/t Lime
280 Kg/t Lime
0
0
5
10
15
Time (h)
20
25
30
Fig. 2. Gold extraction profile at different lime dosage. Feed solids from
mine tailings originally at 1.34 g/t, Au.
Please cite this article in press as: Kasaini, H. et al., Enhanced leachability of gold and silver in cyanide media: Effect ..., Miner. Eng.
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5
40
100
90
80
% Extraction
% Ag Extraction
30
70
60
150 Kg/t Lime
50
100 Kg/t Lime
40
60 Kg/t Lime
Cu
20
Zn
S
280 Kg/t Limestone
30
10
20
10
0
0
5
10
15
Time (h)
20
25
0
30
0
2
4
6
8
10
Time (h)
Fig. 3. Silver extraction profile at different lime dosage. Feed solids from
mine tailings originally at 155 g/t, Ag.
Fig. 4. Behaviour of Cu, Zn, and S during cyanide leaching after alkaline
digestion.
Table 5
Leach efficiency – gold
With regards to Fe and Pb impurities in the feed solids,
it was found that the level of Fe and Pb in the leach solution was too low to affect gold and silver leaching in cyanide media as shown in Fig. 5. In this study, we did not
investigate the effects of other metal-cyanide complexes
on gold recovery during cyanide leahing although researchers have shown the some metal complexes may enhance or
inhibit gold dissolution (Rees and Van Deventer, 1999).
Lime/dosage (kg/t)
% Leach efficiency, Au
Accountability, Au
60
44.99
103.4
100
68.17
107.9
150
65.10
103.9
280
34.43
104.8
60
73.50
96.11
100
89.80
112.87
150
85.70
108.33
280
12.90
104.52
Table 6
Leach efficiency – silver
Lime/dosage (kg/t)
% Leach efficiency, Ag
Accountability, Ag
depends on the host minerals and level of dissolved oxygen.
Elemental sulphur readily reacts with lime to form polysuphides and calcium thiosulphate at moderate temperatures
and cyanide concentrations (Berezowsky et al., 1990).
3.4. Effect of particle size on leaching efficiency of gold and
silver
Fig. 6 underscores the principle that high leach efficiency
could be achieved with particles possessing a large surface
area. As illustrated in Fig. 6, the amount of gold extracted
increased with decrease in particle size. Also, reduction in
particle size tends to minimize the mass transfer resistance
3.3. Effect of metal–cyanide species on gold leaching
6.0
5.0
Solution Metals (ppm)
The principles of leaching gold in the presence of copper, nickel, iron and zinc minerals in cyanide media are
well-known from literature (Breuer et al., 2002). Incomplete gold extraction could be explained by the loss of cyanide due to copper or nickel cyanide complex formation. In
this study, the feed solids contained significant amounts of
Cu, Ag, Fe, Ni, Pb and Zn. These base metals tend to form
cyanide complex ions in the leach solution which in turn
inhibit gold leaching kinetics (Breuer et al., 2002). Fig. 4
shows the leach profile of copper after the pre-treatment
of lead/silver residues with lime. The amount of copper
recovered reached more than 30% after 2 h. Copper leaches
out very fast but its concentration in solution decreases
mainly due to precipitation as a hydroxide at pH values
higher than 8. The leaching profiles of zinc and sulphur
did not follow the copper leaching trend.
4.0
3.0
Fe
Pb
2.0
1.0
0.0
0
2
4
6
8
10
Time (h)
Fig. 5. Behaviour of Fe and Pb during cyanide leaching after alkaline
digestion.
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H. Kasaini et al. / Minerals Engineering xxx (2008) xxx–xxx
80
0.075
% Au Extraction
60
2
R = 0.9577
ks, 1/h
0.050
40
0.025
20
180 - 212 microns
150 - 150 microns
38 - 45 microns
0.000
0
0
5
10
15
Time (h)
20
25
6
30
Table 7
Leach efficiency of gold at different particle sizes
90–106
49.63
101.79
180–212
45.71
97.20
to flow of reactants and products to and from the particle
(see Table 7).
3.5. Modeling: shrinking core sphere
The kinetic models applied in this study have been
applied extensively by researchers in their efforts to interpret leach data (Sohn and Wadsworth, 1979). In this study,
the shrinking core model was used to evaluate the dissolution kinetics assuming an absence of mineral association
effects as shown in Fig. 7. The calculated values of the
leaching rate constants were plotted against the reciprocal
of the particle radii, yielding a linear relationship with
0.96 goodness of fit as shown in Fig. 8.
The linear dependence of the rate constant on the
inverse particle radius supports the surface reaction-controlled shrinking core model.
In the case of silver extraction, re-sizing the particles
below 75 lm did not produce significant improvements in
2
R = 0.9603
63 - 75 microns
90 - 106 microns
2
R = 0.9731
0.2
1-(1-X)
1/3
180 - 212 microns
2
R = 0.9208
0.1
0
0.5
1
1.5
26
31
90
2
2.5
Time (h)
Fig. 7. Plots of 1 (1 X)1/3 versus time at different particle sizes.
80
70
% Ag Extraction
63–75
67.76
103.11
Au (%)
Au (%)
16
21
1/R, 1/mm
Fig. 8. Dependence of rate constant ks on 1/R Particle sizes.
Fig. 6. Extraction kinetics of gold as a function of particle size.
Particle size (lm)
Leach efficiency
Accountability
11
60
50
40
30
20
180 - 212 microns
125 - 150 microns
10
38 - 45 microns
0
0
5
10
15
Time (h)
20
25
30
Fig. 9. Extraction of silver as a function of particle size.
leaching as shown in Fig. 9. It seems that leaching of Ag is
independent of particle size although this observation can
only be made certain if larger particle (>212 lm) are tested.
3.6. Effect of cyanide concentration on gold and silver
extraction kinetics
The effect of cyanide [CN] concentration on the extraction of gold and silver from residual tailings was studied at
40 °C. Gold recovery was enhanced from 39.22% to 68.5%
by adjusting NaCN dosage from 2.5 kg/t to 10 kg/t as
shown in Fig. 10. Table 8 provides a summary of leach data
which is expressed graphically in Fig. 10 (see Table 9).
At 5 kg/t NaCN, the leach curve obtained shows some
deviation from the other curves, a maximum of 19% was
recorded in the first 120 min and no fractions reacted after
8 h where a plateau lying on X-axis is reached that corresponds to the end of reaction, Fig. 11. Furthermore, at
the lowest cyanide concentration (2.5 kg/t), no fractions
of silver are reacted through the experiment (see Table 10).
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% Au. Extraction
H. Kasaini et al. / Minerals Engineering xxx (2008) xxx–xxx
7
80
4. Conclusions
70
This study demonstrated that recovery of gold and silver
from a complex tailings material consisting of zinc refinery
residue, zinc flotation tailings and calcine leach residue is
possible after pre-treating the feed with alkaline solution
(Ca(OH)2). Further milling of particles from 200 to
85 lm showed a positive effect on gold and silver leaching.
However, the effect of alkaline treatment significantly
improved leaching of gold and silver in tailings. This was
attributed to the decomposition of 12% of the Ag-bearing
mineral in the tailings existing as jarosite at a dosage of
100 kg/t lime and 10 kg/t NaCN, 69% Au and 90% silver
were recovered from the composite tailings material.
This project was concluded with a recommendation to
Zincor and Exxaro management to set up a pilot leach
plant for gold and silver recovery from mine tailings. A
commercial plant for reprocessing gold tailings will yield
excellent returns on capital investment at the current gold
price.
60
50
40
30
20
2.5 Kg/t NaCN
5Kg/t NaCN
10
7.5 Kg/t NaCN
10 Kg/t NaCN
0
0
5
10
15
Time (h)
20
25
30
Fig. 10. Gold extraction profile as function of cyanide concentration.
Table 8
Leach efficiency of silver at different particle sizes
Particle size (lm)
Leach efficiency
Accountability
Ag (%)
Ag (%)
63–75
82.11
111.77
90–106
77.40
104.83
180–212
73.50
97.20
Table 9
Leach efficiency of gold at different CN concentrations
NaCN dosage (kg/t)
Leach efficiency
Accountability
Au (%)
Au (%)
2.5
39.22
119.30
5
44.6
96.00
7.5
47.01
86.66
10
68.54
112.04
Acknowledgements
We are thankful to Exxaro technology R&D (Nanne
Vegter) for their technical support and project evaluation.
We also want to express our appreciation to Dr. Sabine
of Pretoria University for analysing our solid samples by
XRD analysis and to Mr. Japie Oberholzer for his analytical work on solutions using the ICP.
References
% Ag. Extraction
90
60
30
2.5 Kg/t NaCN
5Kg/t NaCN
7.5 Kg/t NaCN
10 Kg/t NaCN
0
0
5
10
15
Time (h)
20
25
30
Fig. 11. Silver extraction profile as function of cyanide concentration.
Table 10
Leach efficiency of silver at different CN concentrations
NaCN dosage (kg/t)
Leach efficiency
Accountability
Ag (%)
Ag (%)
2.5
0.46
80.11
5 kg/t
0.59
61.05
7.5 kg/t
40.27
82.66
10 kg/t
80.90
121.91
Adams, M.D. Environmental Impact of Cyanide and Non-cyanide
for Gold Lixiviants. MINTEK, SAIMM Minerals Processing
Design School, Technikon SA Conference Centre, Johannesburg,
1998.
Berezowsky, R.M.G.S., Stikma, J., Kerfoot, D.G.E., Krysa, B.D. Silver
and gold from zinc pressure leach residue. In: Mackey, T.S.,
Prengaman, R.D. (Eds.), Lead–Zinc ’90, Anaheim, California. Proceeding of World Symposium on Metallurgy and Environmental
Control, 119th IMS Annual Meeting, February 18–21. The Minerals,
Metals and Materials Society, 1990, pp. 135–150.
Breuer, P.L., Dai, X., Jeffrey, M.I., 2002. Leaching of gold and copper
minerals in cyanide deficient copper solutions. Hydrometallurgy 78,
156–165.
Crundwell, F.K., Godorr, S.A., 1997. A mathematical model of the
leaching of gold in cyanide solutions. Hydrometallurgy 44, 147–162.
Gabra, G., 1984. A kinetic study of the leaching of gold from pyrite
concentrate using acidified thiourea. In: Precious metals: Mining,
Extraction and Processing. The Metallurgical Society of AIME, Los
Angeles, pp. 145–172.
Grosse, A.C., Dicinowski, G.W., Shaw, M.J., Haddad, P.R., 2003.
Leaching and recovery of gold using ammoniacal thiosulfate leach
liquors (a review). Hydrometallurgy 69, 1–21.
Habashi, F., 1986. In: Principles of Extractive Metallurgy, Pyrometallurgy, vol. 3. Gordon & Breach, New York/London/Paris, p. 479
(reprinted 1992).
Health, A.R., Rumbal, J.A., 1998. Optimising cyanide: oxygen ratios in
gold CIP/CIL circuits. CISRO Division of Minerals, Bentley, WA
6152, Australia. Minerals Engineering 11 (12), 999–1010.
Jackson, E. (Eric), 1986. Hydrometallurgical extraction and reclamation.
Chichester, West Sussex, England: Ellis Horwood; New York: Halsted
Please cite this article in press as: Kasaini, H. et al., Enhanced leachability of gold and silver in cyanide media: Effect ..., Miner. Eng.
(2008), doi:10.1016/j.mineng.2007.12.005
ARTICLE IN PRESS
8
H. Kasaini et al. / Minerals Engineering xxx (2008) xxx–xxx
Press [distributor], ill. (Ellis Horwood series in industrial metals), p.
266.
Liu, G.Q., Yen, W.T., 1995. Effects of sulphide minerals and dissolved
oxygen on the gold and silver dissolution in cyanide solution. Minerals
Engineering 8 (1–2), 111–123.
McLaughlin, J., Agar, G.E., 1991. Development and application of first
order rate equation for modelling the dissolution of gold in cyanide
solution. Minerals Engineering 4 (12), 1305–1314.
Moussoulos, L., Potaminos, N., Kontopoulos, A., 1984. Recovery of gold
and silver from arseniferous pyrite cinders by acidic thiourea leaching.
In: Precious Metals: Mining, Extraction and Processing. The Metallurgical Society of AIME, Los Angeles, pp. 323–325.
Patino, E., Cruells, M.F., Salinas, E., Roca, A., Perez, M., 2003. Kinetics
of alkaline decomposition and cyanidation kinetics of argentian
ammonium jarosite. Hydrometallurgy 70, 153–161.
Prasad, M.S., Mensah-Biney, R., Pizzaro, R.S., 1991. Modern trends
in gold processing – overview. Minerals Engineering 4 (8), 1257–
1277.
Rastas, J.A., Leppinen, J., Hintikka, V., Fugleberg, S., 1990. Recovery of
lead, silver and gold from zinc process residues by a sulfidizationflotation method. In: Mackey, T.S., Prengaman, R.D. (Eds.), Lead–
Zinc ’90, TMS, pp. 193–209.
Rees, K.L., Van Deventer, J.S.J., 1999. The role of metal–cyanide species
in leaching gold from a copper concentrate. Minerals Engineering 12
(8), 877–899.
Sandra, E., Gamini, S., 2004. The effects of dissolved oxygen and cyanide
dosage on gold extraction from a pyrrhotite-rich ore. Hydrometallurgy
72, 39–50.
Sohn, H.Y., Wadsworth, M.E., 1979. Rate Processes of Extractive
Metallurgy. Plenum, New York, pp. 140–148.
Please cite this article in press as: Kasaini, H. et al., Enhanced leachability of gold and silver in cyanide media: Effect ..., Miner. Eng.
(2008), doi:10.1016/j.mineng.2007.12.005
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